The present invention relates to a screen printing device including a work table having a printing position, means on said work table for holding an object, e.g. a substrate, in said printing position, a holder for a screen provided with a mask, a microscope and means for adjusting the relative positions of said table, said screen holder and said microscope.
Such a screen printing device for printing on a substrate is well known in the art, e.g. the Model 155 S Screen Printer manufactured by the PRESCO division of the US firm AMI (Affiliated Manufacturers Inc.). In this known device the work table is movable from an initial position located below an adjustable film holder to a printing position located below the screen mask holder and vice versa. To make a same print on several substrates the device operates as follows. After a reference film--usually the photopositive from which the mask was imaged--has been inserted in the film holder and an auxiliary substrate has been positioned on the work table, the latter is moved into its printing position and at least one trial print is made on this auxiliary substrate. Thereafter, the work table with the auxiliary substrate is brought back into its initial position under the film holder the position of which is then adjusted with respect to the print on the auxiliary substrate with the help of the microscope and of film holder adjustment means until reference strips on the film exactly match with reference strips on the auxiliary substrate. The printed auxiliary substrate is then removed as the device is ready for making a same but definitive print on several substrates. Prior to each such a printing operation reference strips on each of these substrates must however be aligned with the reference strips on the film by means of the microscope and of work table adjustment means.
Summarizing, with the known device, when a same print has to be made on several substrates, first a number of preparatory operations (trial print, film holder adjustment) have to be performed in common for all these substrates, and prior to each definitive printing operation on a substrate a work table adjustment has to be executed. Also several prints may be successively made on a same substrate by proceeding in the above described way for each such print. But because prior to each print a reference film has to be aligned with the reference strips on the substrate, it is clear that the precision with which each print is executed decreases with the number of prints as the alignment errors cumulate.
An object of the present invention is to provide a screen printing device of the above type but which does not need a trial print and wherein the alignment of the mask and the object is enabled in an easy, fast and accurate way.
According to the invention this object is achieved due to the fact that the device further includes a template with reference portions corresponding to reference portions of said mask and that said work table has at least one hole with associated illumination means to simultaneously illuminate said corresponding reference portions of said mask and of said template held in said printing position by said holding means in order to be able to adjust, with the help of said adjustment means, the relative position of said reference portions when viewed through said microscope.
In this screen printing device, first the template is positioned in the printing position of the work table and held therein by the holding means. Then the screen holder carrying the mask is brought in the immediate proximity of the template in such a way that both reference portions of the mask and corresponding reference portions of the template are located in the center of an illuminated hole of the work table and may be simultaneously viewed through the microscope. With the help of the adjusting means the reference portions of the mask are then aligned with those of the template so that this mask is then correctly positioned with respect to the printing position on the work table. The device is then ready for executing a printing operation on a substrate when the latter is substituted for the template in the printing position. Hence, no trial print is necessary and once the position of the mask has been fixed successive substrates may be printed without any further adjustment. This obviously increases the speed of production and ensures a correct and constant alignment for all the substrates. In case a number of consecutive prints with different masks has to be made on a same substrate, the previously described alignment has to be executed between each mask and the template prior to each print. But because each of these alignments is performed with respect to the same references on the template, possible positioning errors never cumulate so that the accuracy obtained is very high.